US 7469841 B1
A self-contained thermal actuator positions the piston return member inside the actuator cup. The inventive configuration eliminates the guide and diaphragm seal that closed the open end of the wax filled cup in the typical thermal actuator configuration. The piston and an O-ring seal seated in an annular recess defined by the piston contain the thermally responsive wax. A return member within the cup and engaged between axially spaced, radially overlapping shoulders on the cup and piston biases the piston toward its pre-actuation position. The return member may be a spring or an elastomeric O-ring that also seals the thermal actuator against intrusion of contaminants from the use environment. The piston is closely received in the cup for guided axial reciprocation therein.
1. A thermal actuator comprising:
a rigid cup having a first shoulder, a longitudinal axis, a closed bottom portion defining a cavity and an open top portion;
a quantity of thermally responsive wax in said cavity;
a plunger disposed in said cup and extending from said thermally responsive wax to at least said open top portion for axial reciprocation therein, said plunger including a second shoulder;
a seal arranged to contain said thermally responsive wax in said cavity; and
a return member within said cup and engaged between said first shoulder and said second shoulder to bias said plunger toward said thermally responsive wax at all positions of said plunger.
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14. A thermal actuator comprising:
a cup defining a bore extending from a closed bottom portion filled to an intermediate level with thermally responsive wax to a top portion comprising a wall from which extends a radially inward projecting lip defining a first annular shoulder and an axial opening;
a piston arranged in said bore and including a first end defining an annular recess adjacent said thermally responsive wax, a second end including a head adjacent said axial opening and a radially projecting second annular shoulder intermediate said head and said first end, said second annular shoulder radially overlapping said first annular shoulder in spaced axial relationship;
a seal seated in said annular recess and radially compressed between said cup and said piston to contain said thermally responsive wax in said bottom portion; and
an annular elastomeric polymer member engaged between said first and second annular shoulders to bias said piston toward said thermally responsive wax and a pre-actuation position,
wherein said thermally responsive wax expands in response to an increase in temperature ΔT to generate a force F on said piston and seal to compress said annular elastomeric polymer member between said first and second annular shoulders to move said piston axially away from the closed bottom portion of the cup to an actuated position.
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1. Field of the Invention
The present invention relates to a thermally actuated control device, particularly of the type wherein a thermally responsive wax generates a force to move an actuator in the form of a push rod or the like.
2. Description of the Related Art
Actuators of this type are often used for thermal control valves, such as described in U.S. Pat. Nos. 2,873,633, 4,036,433 and 5,176,317. Typically, such actuators are designed to produce a predetermined actuating movement in response to a change in temperature. One typical configuration for a thermal actuator includes a rigid cup filled with the thermally responsive wax, a resilient diaphragm wax seal covering the wax, a rigid guide member covering the diaphragm wax seal, and an actuator rod or piston received in the guide member to bear on a plug extending from the wax seal. An increase in temperature ΔT causes the thermally responsive wax to expand, generating a force against the diaphragm wax seal, plug and piston to do some work. An actuator of this type can be used to open or close a valve, for example. The typical thermal actuator installation also includes a return spring arranged to bias the piston, plug and diaphragm toward their pre-actuation (return) position. The return spring ensures that upon a decrease in temperature the shifted valve or element returns to its pre-actuation position. The necessity for an external biasing member complicates assemblies utilizing the wax filled thermal actuator and precludes provision of a self-contained actuator of this type. The present inventors have recognized a need to provide a thermal actuator of the thermally responsive wax type which does not require an external biasing element. Such a thermal actuator simplifies assemblies using the device and expands possible applications for the actuator.
The present invention relocates the return member for a thermal actuator inside the rigid cup and eliminates the guide and diaphragm seal typical of the prior art. The piston of the inventive thermal actuator is configured to fill the top portion of the cup. A seal between the piston and the cup contains the thermally responsive wax in the bottom of the cup. The inside surface of the cup and the outside surface of the piston cooperate to guide the reciprocal axial movement of the piston between pre-actuation and actuated positions.
The inventive thermal actuator eliminates the separately manufactured guide member of the typical actuator design. Further, a simple O-ring seal replaces the more complex diaphragm seal and its associated plug. One benefit of the inventive design is a more direct force delivery from the expanding wax to the piston due to elimination of the intervening diaphragm seal and plug of the prior art. O-ring seals are standard elements whose mechanical behavior and sealing characteristics are well understood. Therefore, an effective wax containment seal can be accomplished at reduced cost compared to the previous diaphragm type seal.
The return member may be a second O-ring-shaped elastomeric member engaged between an annular shoulder on the cup and an annular shoulder on the piston to return the piston to its pre-actuation position when the thermally responsive wax cools. This configuration is particularly suited to a use environment for the thermal actuator that requires relatively short plunger travel. Alternatively, the return member may be a conventional coil spring or a stack of Belleville-type washer springs. Other configurations and materials for an internal return member may be compatible with the present invention.
The invention is particularly useful in applications requiring a miniature, self-contained thermal actuator. The disclosed thermal actuator configuration provides a simple, durable, reliable and relatively inexpensive thermally responsive actuator.
A first embodiment of a thermal actuator according to aspects of the present invention will now be described with reference to
The bottom portion 16 of the cup is filled with a predetermined quantity of thermally responsive wax 28. The thermally responsive wax 28 is selected to provide a desired actuation force F by expansion in response to a known increase in temperature ΔT. As is known in the art, thermally responsive wax can be formulated to expand to generate the actuation force F over a broad range of temperatures.
A piston 30 (which also may be referred to as a plunger herein) is arranged in the longitudinal bore 14 of the cup 12 for axial reciprocation between pre-actuation and actuated positions. The piston 30 extends from a first end 32 adjacent the thermally responsive wax 28 to a second end 34 having a head 36. The piston head 36 includes an actuation surface 38. The actuation surface 38 of the illustrated piston 30 is a conical surface centered on a longitudinal axis A of the piston 30. The conical actuation surface has a shallow angle α of approximately 7° relative to a perpendicular P to the longitudinal axis A. Piston diameters D5 and D6 cooperate with longitudinal bore diameters D1 and D2 to guide the piston 30 during axial movement.
The piston 30 includes a radially outwardly projecting second annular shoulder 40 having a diameter D6. The second annular shoulder 40 is configured to radially overlap the first annular shoulder 26 (provided by the inwardly formed lip 24) in axially spaced relationship. A return member 42 in the form of an elastomeric O-ring is engaged between the first and second annular shoulders 26, 40 to bias the piston 30 toward the thermally responsive wax 28. As best shown in
The first end 32 of the piston defines an annular recess 44 in which is seated an O-ring seal 46. The O-ring seal 46 is selected with a thickness T that will be radially compressed between D7 of the piston 30 and the inside surface 48 of the longitudinal bore to contain the thermally responsive wax 28 below the piston 30. Force F generated by the expanding wax is delivered to the first end piston and seal 46 to move the piston toward the open end portion 20 of the cup, e.g., toward an actuated position. This piston movement compresses the return member 42 and moves the actuation surface 38 of the head of the piston above the formed lip 24 of the cup to an actuated position (shown in dashed lines on
The configuration of the return member 42 and/or the properties of its material can be altered to control movement of the piston 30. For example, changing the durometer of the return member material will have an effect similar to placement of a weaker or stronger spring in the assembly. Material durometer also affects the speed of movement of the piston 30. Another variable in piston movement for the thermal actuator 10 is the configuration of the piston groove 54 in which the return member 42 is seated, as well as the space 50 surrounding the return member 42. The space 50 around the return member 42 accommodates its elastic deformation during piston actuation. In thermal actuator 10, this space is provided by the diameters D3 and D4 of the top portion 20 of the cup as shown in
As best shown in
While a preferred embodiment of the foregoing invention has been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.